Late-stage Deccan eruption from multiple shallow magma chambers through vertical flow along fissures: Insights from magnetic fabric analysis of the Pachmarhi dyke swarm

The Pachmarhi dyke swarm, located in the eastern part of the Narmada-Satpura-Tapi dykes belonging to the Deccan Continental Flood Basalt, are studied using the Anisotropy of Magnetic Susceptibility (AMS) technique. This research aims to determine the direction and sense of magma flow within the dykes, providing insights into the depth, number, and location of magma chambers, as well as the geodynamics of their plumbing system. Petrography and rock magnetism analyses revealed a mixture of high- and low-titanium magnetite particles, predominantly of pseudo-single domain nature (with a smaller proportion of multi-domain dominated) grains are primary remanence carriers. We identified four distinct types of magnetic fabric (I-IV) within the Pachmarhi dykes. The K₁-axis being parallel to the dyke plane, and the intersection of the imbrication angle of magnetic foliation (for oblate fabric) and magnetic lineation (for prolate fabric) was used to discern the direction of magma flow. This analysis revealed multiple trends of magma flow, ranging from vertical/sub-vertical to inclined. The flow fabric provides valuable information about the presence of multiple shallow sub-crustal magma chambers. This interpretation aligns with prior independent gravity and 3-D density modelling studies, which indicates the presence of dense mafic magma bodies at depths of 4 to 8 km along the Narmada-Tapi intraplate rift zone. These findings are similar to those observed in the Nandurbar-Dhule dyke swarms in the western region of the Narmada-Satpura-Tapi dykes. Consequently, we can infer that the emplacement of dykes in the Pachmarhi region of the Narmada-Son-Lineament, which likely served as feeders for the late-stage Deccan volcanism, was primarily facilitated by a “polycentric flow” mechanism. In this process, magma was injected vertically from multiple shallow magma chambers through crustal fissures, potentially feeding into the late-stage Deccan flow units, such as the Ambenali or Mahabaleshwar Formations.